Conventional transport aircraft typically utilize pneumatic, hydraulic, and electric power from main engines to support various aircraft systems during flight. In addition, conventional transport aircraft typically utilize pneumatic and electric power from onboard auxiliary power units (APUs) to support aircraft systems during ground operations. Aircraft air conditioning systems (also known as environmental control systems, or ECSs) are typically the largest secondary power users on commercial transport aircraft. Some conventional ECSs use high temperature/high pressure air extracted from the engine compressor stages (bleed air). In such a pneumatic bleed architecture, hot pressurized air is extracted from the compressor section of the engine, cooled, and then routed through ducting to pneumatically power utilization equipment such as the ECS. The APU typically drives an air compressor to provide a source of air for the utilization equipment at certain times during flight and on the ground before the main engines are started. During a typical engine start, pressurized air provided by the APU compressor (or the other engine if it is running) is routed through a duct to an air turbine starter on each engine. During engine start, the supply of air to other utilization equipment is temporarily suspended until the engine is successfully started, to allow the entire air flow from the source to be utilized to start the engine. When pneumatic bleed air is extracted from an engine, this work (performed by the engine) does not help to produce thrust, so engine fuel economy is adversely affected. However, during some phases of engine operation it is necessary to reduce pressure inside the engine by extracting bleed air to maintain proper operability margin.
A traditional pneumatic architecture would not be cost effective in a smaller, lightweight aircraft that must satisfy new FAA requirements for passenger fresh air flow (which will result in greater ECS power consumption and reduction in airplane fuel consumption compared with current production aircraft). There have been few advances in pneumatic technology to serve as enablers for a substantially improved pneumatic aircraft architecture.
Modern aircraft are beginning to employ an all-electric architecture. An all-electric architecture eliminates pneumatic (bleed) air extraction from the engines and APU. An all-electric architecture uses large electrical generators driven by the engines, and large generators on the APU, to produce electrical power sufficient to drive major system utilization equipment. These generators produce variable frequency AC electrical power, in which the frequency of the power tracks the speed of the engine rotor to which the generator is connected. A majority of the AC power is immediately rectified to produce DC power which is supplied to large motor controllers that operate the motors in the ECS equipment, hydraulic motor pumps, and other equipment. These large motor controllers offer continuously variable motor control, good motor torque characteristics, high efficiency, soft start, and low inrush current. However, they are large, heavy, and require a special liquid cooling system to address the high densities of heat that are generated during power conversion. The all-electric architecture can generally be characterized as being efficient for long flights, but very complex. Consequently, such an all-electric architecture may not be the best approach for a smaller aircraft with a lower target cost and shorter mission range.
A traditional all-electric architecture ceases to be cost effective or meet cost and weight targets when it is scaled down to meet the needs of a much smaller aircraft. In particular, the large motor controllers and power conversion equipment required to operate major systems on the aircraft from power sources that produce variable frequency AC power are bulky, heavy, costly, and require a special liquid cooling system. In addition, many smaller motor loads located throughout the aircraft require motor controllers to be compatible with variable frequency AC power, resulting in added complexity, cost and weight. Although the efficiency gained for long flights is better for the all-electric architecture than for a traditional pneumatic system, the all-electric architecture has not been demonstrated to be the most cost effective approach for a much smaller aircraft with shorter range.